Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Collaboration revenue	$ 90,000		$ 90,000
Type of Revenue [Extensible List]	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember
Operating expenses:
Research and development	$ 68,980	$ 55,900	$ 118,250	$ 101,100
General and administrative	43,167	14,954	72,016	27,234
Total operating expenses	112,147	70,854	190,266	128,334
Loss from operations	(22,147)	(70,854)	(100,266)	(128,334)
Interest income, net	5,137	672	8,666	1,379
Other expense, net	32	(20)	24	(24)
Net loss	$ (16,978)	$ (70,202)	$ (91,576)	$ (126,979)
Net loss per share—basic and diluted	$ (0.36)	$ (1.88)	$ (2.02)	$ (3.40)
Weighted average number of common shares outstanding—basic and diluted	46,541,716	37,361,129	45,439,666	37,315,393
Comprehensive loss:
Net loss	$ (16,978)	$ (70,202)	$ (91,576)	$ (126,979)
Other comprehensive items:
Unrealized gain (loss) on marketable securities	40	14	(116)	35
Total other comprehensive gain (loss)	40	14	(116)	35
Total comprehensive loss	$ (16,938)	$ (70,188)	$ (91,692)	$ (126,944)